Thermal spray deposition of surface coatings has been used to form hard coatings on machine tools, thermal barriers on refractory surfaces, and hydrophobic layers on medical tools, paper mill rollers, or pharmaceutical equipment. In thermal spray deposition, molten droplets of a coating material impinge upon and adhere to a target surface. Ideal deposition conditions include substantial uniformity of droplet composition, size, velocity, and temperature. Such ideal deposition conditions cannot be achieved with current spray technologies. In particular, uniform droplet temperature has proven very difficult to achieve.
In the production of ceramic structures for gas turbines or for optical applications, crystalline ceramic powders are subjected to hot isostatic pressing to eliminate voids and porosity, thereby enhancing structural properties. However, semi-pyrolized or un-melted inclusions due to varying particle sizes can lead to inhomogeneities including voids that harm the properties of the end point ceramic body. Ceramics being generally brittle have mechanical properties strongly affected by flaws. It is also important to have highly uniform powders for optical applications, which are rapidly degraded by even a small percentage of defective regions. Rosenflanz et al., Bulk glasses and ultrahard nanoceramics based on alumina and rare-earth oxides, Nature, 430, 761-764 (12 Aug. 2004), proposed a method for making fine grained ceramics that involves making an amorphous glass phase by aggregation of molten particles in a coolant, deforming the glass to eliminate pores, and then crystallizing the glass. The resulting crystalline ceramic should have very fine grains. However, final mechanical and optical properties are highly dependent on the uniformity of the molten particles aggregated in the coolant.
Traditionally, molten droplets either for surface coating or for powder production have been formed by injecting particles of a source material between about ten microns (10 μm) and about one hundred microns (100 μm) diameter to a plasma jet. In most known thermal spray devices, the plasma jet is generated by a DC electric arc housed in an plasma torch, and the particles have been injected to the plasma jet external to the torch. Particles injected to the plasma jet external to the plasma torch necessarily enter the plasma jet off-axis, thereby introducing local non-uniformity to the entrainment of the particles in the plasma jet. The local non-uniformity of entrainment is exacerbated by highly chaotic behavior of the plasma jet, which can fluctuate in velocity by a factor of approximately two (2) at frequencies on the order of several thousand fluctuations per second (kHz). Moreover, entrainment of ambient gas into the plasma jet at the plasma torch exit results in a highly non-uniform radial and axial temperature profile external to the plasma torch. Accordingly, particles injected to the plasma jet external to the plasma torch are not uniformly entrained in or heated by the plasma jet. Thus, inhomogeneous particles are produced.
In order to improve the uniformity of product particles, efforts have been made to create axial feed arc-plasma torches. In particular, both Mettech, Inc. of Vancouver, Canada and Sulzer Metco of Switzerland have invested years of engineering effort toward developing reliable and commercially practicable axial feed arc-plasma torches. It is noted that internal structures required for axial injection are difficult to make compatible with internal structural constraints related to establishing and maintaining a DC arc plasma jet. DC electric arc plasma generation is a highly unstable process that can be disrupted by structural protrusions or foreign particles. Moreover, the high temperatures of the electric arc plasma impose strict limits on the design of injectors. Accordingly, known DC arc plasma torches are designed to have substantially smooth interior surfaces. Additionally, DC arc plasma torches require substantially particle-free interior volumes in the region where the arc is produced.
High-speed oxy-fuel flame jets also have been used to melt the powder that is sprayed onto the surface of interest. Although combustion torches often provide axial injection, such torches do not produce high enough temperatures to melt ceramic materials useful for thermal barrier coatings. Combustion torches also have high operational costs, and can produce coatings of notably varying chemical composition, because such torches require large quantities of substantially chemically pure fuel gases.
In either arc plasma or combustion torches, relatively high velocity of the plasma jet is required to adequately entrain and transport transversely injected particles. Accordingly, regardless of injection location, the majority of particle heating typically takes place exterior of the torch, where very non-uniform conditions prevail. When the non-uniformly heated particles impact on the surface to be coated, the particles cool, shrink, and adhere at varying rates to varying final thicknesses and bond strengths. More importantly, unmelted particles may be deposited which do not flatten leading to non-uniform structure. In the case of solution plasma spray, partially pyrolized or partially evaporated material can be deposited and subsequently shrink creating voids. All of the preceding non-uniformities of particles will lead to inhomogenous microstructures of the coating. Thus, arc plasma or combustion torches inherently provide a significant degree of non-uniform coating properties.
A third class of apparatus, highly amenable to axial injection, is the RF plasma gun developed by Boulos et al. RF plasma is formed by an induction coil producing a relatively low temperature, low velocity plasma. However, as taught by Boulos' 1985 article “The inductively coupled R.F. (Radio Frequency) Plasma,” Pure Appl. Chem. 57, p. 1321, these torches are useful only for making powders or metal coatings from relatively large feedstock.
Therefore, there is a need in the industry for improved apparatuses and methods for generating substantially uniform particles by plasma material processing.